Lower molecular weight alcohols and ethers such as isopropanol (IPA) and diisopropyl ether (DIPE) are in the gasoline boiling range and are known to have high blending octane numbers. In addition, by-product propylene from which IPA and DIPE can be made is usually available in a fuels refinery. An important aspect of research in the petroleum industry relates to processes to produce high octane lower aliphatic alkyl ethers as octane boosters and supplementary fuels.
The catalytic hydration of olefins, particularly C.sub.3 and C.sub.4 olefins, to provide alcohols and ethers is a well-established art. Representative olefin hydration processes are disclosed in U.S. Pat. Nos. 2,262,913; 2,477,380; 2,797,247; 3,798,097; 2,805,260; 2,830,090; 2,861,045; 2,891,999; 3,006,970; 3,198,752; 3,810,848; 3,989,762, among others.
Olefin hydration employing medium pore and large pore zeolite catalyst is a known synthesis method. As disclosed in U.S. Pat. No. 4,214,107 (Chang et al.), lower olefins, in particular propylene, are catalytically hydrated over a crystalline aluminosilicate zeolite catalyst having a silica to alumina ratio of at least 12 and a Constraint Index of from 1 to 12, e.g., acidic ZSM-5 type zeolite, to provide the corresponding alcohol, essentially free of ether and hydrocarbon by-product. Acid resin catalysts such as "Amberlyst 15" may also be used for hydration of light olefins.
The production of ether from secondary alcohols such as isopropanol and light olefins is known. As disclosed in U.S. Pat. No. 4,182,914, DIPE is produced from IPA and propylene in a series of operations employing a strongly acidic cationic exchange resin as catalyst. Recently, processes for the hydration of olefins to provide alcohols and ethers using zeolite catalyst such as ZSM-5 or zeolite Beta have been disclosed in U.S. Pat. Nos. 4,214,107 and 4,499,313 to Bell et al.; and U.S. Pat. Nos. 4,757,664, 4,857,664 and 4,906,187 to T. Huang. These patents are incorporated herein in their entirety by reference. One of the advantages in using zeolite catalyst for hydration and/or etherification of light olefins is the regenerability of the catalyst. Where resin based catalysts can decompose at the high temperatures required to remove deactivating amounts of carbonaceous deposits, zeolite catalysts remain thermally stable and can be regenerated oxidatively or in contact with hydrogen.
The hydration and etherification of lower olefins such as propylene to produce IPA and DIPE over a fixed bed of shape selective zeolite catalyst is generally carried out in liquid phase employing a feedstream comprising water and propylene at temperatures in excess of 200.degree. F. and high pressure, preferably above 1000 psi. The maximum per pass conversion of propylene to DIPE is about 65 wt %. While attempting to maximize the rate of conversion, process conditions are selected to also reduce the more disadvantageous reactions which can occur during the process that could compromise the process advantages. These adverse reactions include the oligomerization of propylene, the formation of deactivating amounts of coke and carbonaceous deposits on the catalyst and the hydrothermal attack of water on the catalyst. These adverse reactions tend to find favor with increasing temperature and concentration providing a challenge to workers in the field to control reactor temperature, particularly at start-up when components may be present in high concentration.
In view of the foregoing problems relating to adverse reactions in the production of IPA and/or DIPE over fixed bed zeolite catalyst, starting up a hydration/etherification reactor is a particularly difficult problem. The fresh reactants, water and propylene, alone or in a mixture, exhibit high heats of adsorption on zeolite catalyst to the extent that on start up the fresh feed exotherm can cause severe local overheating of the catalyst bed. Hydrothermal attack by water on the catalyst destroys the structural integrity of the catalyst producing an excessive amount of generally ineffective fine particles. These fine particles are not only less catalytically effective but they complicate the subsequent removal of catalyst from the reactor. Propylene, added under typical start up conditions, can also experience local overheating providing conditions that promote unwanted oligomerization of propylene to produce undesirable by-product such as propylene dimer. These reactions of propylene can also lead to the formation of carbonaceous deposits on the catalyst and coke formation.
Starting up olefin hydration and etherification reactors containing fixed beds of zeolite catalyst present unique problems not encountered in commercial zeolite catalyzed conversion processes. Zeolite catalyzed processes such as the conversion of methanol to gasoline (MTG) or the oligomerization of olefins to higher hydrocarbons, the Mobil olefins to gasoline process (MOG), are vapor phase processes carried out at high temperature with feedstreams introduced as gases. Olefin hydration and etherification such as the zeolite catalyzed DIPE process are liquid phase processes that must be carried out at moderate temperatures and relatively high pressures. In the latter case, the likelihood of localized overheating and the consequent development of adverse side reactions during reactor start up are problems of substantial proportions that go beyond known commercial practice and have remained largely unresolved in the art heretofore.
Commercial start-up procedures for MTBE, methyl tertiary butyl ether, production where water is used to wet the catalyst would cause high temperature steaming for the DIPE system and lower activity due to water filled pores.
It is an object of the present invention to provide an improved process for the hydration of olefins using a fixed bed of zeolite catalyst particles.
It is a particular object of the present invention to provide a method for starting up an olefin hydration reactor containing a fixed bed of zeolite catalyst particles that avoids reactions deleterious to the efficacy of the catalyst.
A further objective of the invention is to provide an improved start up method for the zeolite catalyzed process for the conversion of propylene and water to IPA and/or DIPE.